FIG. 1 is a front view illustrating the structure of a conventional metal halide lamp of the vertical type. An arc tube (1) made of a quartz glass has a pair of main electrodes (2a) and (2b) at both ends of the interior thereof and the interior thereof is filled with an inert gas, mercury and a metal halide. An outer tube (3) covers the arc tube (1) and the interior thereof is filled, for example, with a nitrogen gas. A base (4) is disposed at the upper end of the outer tube (3) and electrically connected to the electrodes (2a) and (2b). A heat-insulating coating (5) is provided at the lower end of the arc tube and formed, for example, of a zirconia coating.
The lamp constructed in this way is normally positioned for use with the base (4) directed upward, but in such a lighted state the lower end of the arc tube (4) is cooled due to a convection of the gas within the arc tube (1) and convection of nitrogen with the outer tube (3) and becomes the coldest part. Since the vapor pressure of the metal halide changes dependent on the temperature of said coldest part, the luminous efficiency also depends upon the temperature of the coldest part. As a means for raising the temperature of said coldest part, there has been used a method of increasing the thickness of the zirconia coating or increasing the width of the coating. However it has been found that, although the use of the heat-insulating coating (5) raises the temperature of said coldest part, the temperature of the coldest part is still low and the luminous efficiency is bad particularly with the outer tube (3) the interior of which is filled with something like the nitrogen gas.
On the other hand, for the purpose of raising the temperature of said coldest part, an increase in thickness of the coating has the disadvantages that it is difficult to maintain the stable characteristic of the coating and the coating peels off in the heat cycles which occur during the lighting and so on. Also an increase in width of the coating increases the proportion of visible light radiating from the electric arc which is absorbed due to the influence of the bonding agent added to zirconia and other coating materials. Alternatively the temperature distribution of the arc tube becomes uneven in the vicinity of the place there where the heat-insulating coating is disposed. Thus there has been the disadvantage that a sufficient improvement in efficiency can not be realized.
As another conventional example of the heat-insulating member, for example, Japanese Patent Publication No. 2867/1966 (U.S. patent application Ser. No. 368,471, May 19, 1964) discloses an arrangement in which a metallic end cap is disposed on one end part of an arc tube and the gap between said end cap and the outer wall of the arc tube is filled with a refractory fibrous material to increase the temperature at the end part of the arc tube. In said arrangement, however, one part of the output of visible light from the electric arc formed within the arc tube during the lighting is absorbed by said refractory fibrous material. Alternatively even though a greater part of the visible light is reflected from said end cap into the electric arc, it is absorbed by a metal halide existing in the electric arc or a disassociated metal. Thus it has not been a desirable heat insulating arrangement from the standpoint of improvement in efficiency.
Further as a conventional example of a separate heat insulating member, for example, Japanese Patent Publication No. 2866/1966 (U.S. patent application Ser. No. 323,672, Nov. 22, 1963) discloses a technique for increasing the temperature of an arc tube by providing a glass tube for enclosing the arc tube along with a shield plate. By said technique, however, the heat insulating effect is certainly improved but the highest temperature of the arc tube is simultaneously raised because the arc tube as a whole is thermally insulated. Thus it is not desirable from the standpoint of the lifetime characteristic of the lamp. Also since the axial temperature difference on the tube wall of the arc tube (the difference between the coldest temperature and the highest temperature) is not improved, the axial unevenness of light emitted from the electric arc remains unimproved. Thus a sufficient improvement in efficiency can not be realized.